A. Field of Invention
The present invention relates to light sources which exhibit lumen depreciation over their operating lives and, in particular, to methods, apparatus, and systems for operating such light sources to compensate, at least partially, for such lumen depreciation, reduce costs, and save energy.
B. Problems in the Art
Most high intensity discharge (HID) lamps exhibit what is called lamp lumen depreciation (LLD) characteristic. HID lamps include, but are not limited to, fluorescent, sodium (HPS), metal halide (MH), mercury vapor (HgV), and low pressure sodium (LPS). Each of these specifically mentioned types of HID lamps require a ballast transformer that regulates the operating and starting voltage at the lamp.
One definition of lumen depreciation or LLD is the gradual decline in a source's light output over operation time. Light output from the light source does not stay constant if operated at rated operating wattage. Due to several factors, primarily blackening of the inside of the arc tube from precipitation of chemicals and erosion of electrodes, light output usually drops as the lamp is operated. This characteristic is well known in the art. For example, a typical 1500 W MH lamp can lose up to around 50% of its light output over a typical 3000 hour cumulative operation life. See, for example, the graph of FIG. 1. Interestingly, in some lamps (including many MH lamps), lumen depreciation occurs most rapidly during the first several hundred hours of operation (e.g. 20% light loss). The rate of depreciation slows thereafter (e.g. sometimes on the order of another 10% loss for each subsequent 1000 operating hours). But cumulatively, relative to initial light output, the lamp will lose about one-half of its light-producing capacity by end of its rated life.
Manufacturers give HID lamps a rated operating wattage (ROW). ROW is the recommended wattage to operate the lamp. Manufacturers do not recommend operation substantially over ROW, as they indicate a belief it could cause failure or, at least, reduce useful life of the lamp. They indicate operation at the ROW will provide the most efficient and long-lasting operation of the lamp.
Operation substantially under ROW is also not recommended because starting the lamp can be a problem. The arc may simply drop out without sufficient power. Also, operation too far below rated wattage can materially affect efficacy of the lamp. It can also reduce light output so much as to make use of the lamp impractical for its cost. Other possible detrimental effects on the lamp or its light output are believed possible.
For example, manufacturers' generally recommend a 1500 W NH lamp not be operated at more than 1750 W (about 15 to 20% above ROW) or less than 1000 W (about 30 to 35% below ROW).
Although LLD is different for each lamp (even lamps of the same type, ROW, and manufacturer), the characteristic is well known and is fairly predictable for the same type of lamps. LLD for a particular lamp can usually be found in the technical information available from manufacturers. Sometimes LLD is expressed in terms of a multiplier factor (lumen depreciation factor or LDF) that can be used in illumination calculations to predict reduction in the light output of a lamp over a period of time caused by lumen depreciation. The LDF is usually determined by dividing the maintained lamp lumens by the published initial lamp lumens, usually yielding a value of less than 1. The LDF therefore is used in the industry as an indication of how much light loss from LLD can be expected for a lamp over its operating life.
Other factors, in addition to lumen depreciation, can contribute to what is called total light loss factor for a light fixture. Some of these factors do not involve operation of the lamp itself, such as ballast factor, ambient fixture temperature, supply voltage variation, optical factor, and surface fixture depreciation. But LLD is a significant contributor to total light loss factor.
A particular example of the LLD problem can be given in the context of sports lighting. MH lamps are commonly used, usually having ROWs on the order of at least 700 or 800 watts, and more frequently 1,000 watts, 1,500 watts, or higher. Lamp ROW gives an indication of how much electrical power is needed to run them at a specified operating voltage. Light or lumen output of a lamp is a function of wattage. For example, a 1500 W MH lamp (product ordering code MH1500/U) from Philips Lighting, a division of Philips Electronics N.V. outputs about 155,000 lumens initial and 124,000 mean lumens when operated at 1500 W. A 1000 W MH Philips lamp (product ordering code MH1000/U) outputs about 105,000 lumens initial and 66,000 mean lumens. Wide area, outdoor lighting systems presently tend to favor 1000 W to 1500 W lamps because of the larger light output. Lamps over 1500 W are becoming increasingly available and used.
With reference to FIG. 5, wide area outdoor lighting, such as is used in sports field lighting to illuminate outdoor sports fields, typically utilizes several sets or banks 16 of HID luminaires 14 (each including an HID lamp 10) to illuminate not only field 24, but a volume of space above the field, to make it playable for the players and watchable from spectator stands 26 for different sports. The conventional approach is to mount lighting fixtures 14 in sets 16 on tall poles 18. A common type of lighting fixture or luminaire 14 includes a relatively high wattage high intensity discharge (HID) lamp 10 mounted in an aluminum reflector 12. Electrical power 22 is supplied via conductive cables to ballasts in ballast boxes 20, which distribute electrical power to each lamp 10. Most times a light level is specified for the field. The lighting must be designed to meet such light levels by the selection of number of fixtures (based on light output from such fixtures, which is primarily dependent upon the lamp selected), the size and type of reflector, and their aiming directions to the field. These issues are well known in the art, as are a variety of methods of selection and design of lighting configurations to meet a specified light level. Recommended levels of illumination exist for visibility and safety for various size, shape, and type of sports fields. Light levels that are too low raise not only visibility issues, but also safety considerations. For example, low or uneven light levels can make it difficult for a player to see a fast moving ball.
Theoretically, there can be almost an infinite number of ways to light a field to a specified light level. For example, a thousand fixtures could be elevated on poles or other superstructures and densely packed together encircling the field. However, this is usually impractical. Not only would the cost of that many fixtures (including lamps) be high, the cost of structures to elevate them would be likewise. The cost of maintenance would also be high. And, over time, the cost of energy to operate them would be high. Since many, if not most, athletic field lighting systems are funded by the public or non-profit organizations (e.g. schools, municipal recreation departments, private recreation leagues), cost is a major factor in selecting such lighting.
Therefore, it is conventional to try to minimize the structure used to elevate fixtures and also minimize the number of fixtures for a lighting application to reduce both capital and operating costs. This has driven HID lamp manufacturers to develop more powerful lamps so that each fixture can output greater amounts of light energy to, in turn, allow less fixtures to meet a specified light level for a field. Less fixtures allows less elevating structure (e.g. less poles). For example, it has been reported that capital costs for installations with 1000 W fixtures can be at least 30 percent higher over installations with 1500 W fixtures.
However, as previously discussed, MH lamps (and most HID lamps), have an initial light output at rated wattage (after an initial “break in” period), but then, over the life of an HID lamp, the lamp usually slowly loses lumen output from LLD, even if that same level of electrical power or rated wattage is supplied. The practical effect of lumen depreciation is that, by the latter part of normal operating life of the lamp, its light output is a fraction of its starting output. If used in a system which requires a specified light level or output from the light source, the light source may have to be replaced early because it alone, or in combination with other lamps of similar reduced output, may render the light level to the target unacceptable.
One way of dealing with LLD is to do nothing. Even though the LLD characteristic will most likely result in a drop in light level from the light source, in many lighting applications it is not considered worth addressing. The drop in light level over time is simply accepted, or is not deemed significant enough, functionally or economically, to act upon. With HID lamps, the initial rapid drop-off is usually no more than 10-20%. And, subsequent light loss from LLD tends to proceed at a slower rate after that rapid initial lumen depreciation period. The lumen drop-off may not even be noticeable to most observers. However, in applications where light output is specified for a light source or for the area or target to be lighted by the light source, as is the case for wide area sports lighting, lumen depreciation can be a significant problem. As stated, in sports lighting, if light levels drop too much, it can not only be more difficult for spectators to see the activity on the field, it can become dangerous for players. Thus, doing nothing to compensate for LLD is not satisfactory for such lighting applications.
A second approach to the LLD issue is to replace lamps well prior to end of predicted operating life. For example, some specifications call for all lamps to be replaced at 40% of predicted life. While this tries to deal with the light loss from LLD, replacing lamps early during expected life span adds significant cost to the lighting system, and wastes potential usefulness of some lamps.
If lumen depreciation is dealt with in sports lighting, however, the most common way is a third approach, as follows. The designs essentially engineer into the system an excess amount of light fixtures (and thus additional lamps) in anticipation of light output drop-off caused by at least the first, rapid 10-20% depreciation, so that after about 100-200 hours of operation, the light output is at about the specified level for the particular application. These designs conventionally specify that the lamps be operated at rated operating wattages. The excess fixtures, and the higher energy use, add cost to the system (capital and energy) compared to less fixtures (and less lamps), but try to compensate (at least initially) for light loss from LLD. Also, this way of dealing with LLD does not add additional components, and the associated cost, to the lamps, or to their luminaires or electrical circuitry. It simply adds additional conventional lamps and fixtures. Therefore, a light designer typically selects a type and number of conventional HID lamps and fixtures that cumulatively may initially exceed the lighting requirements because the designer knows that, over time, the lumen depreciation will drop the lighting level below recommended standards. However, after the initial rapid LLD period, lumen levels decrease (somewhat slowly), but will normally gradually move below the recommended light levels. This latter LLD (after the first more rapid LLD) is many times not adequately accounted for in system design, or is ignored.
Designers may use a lumen depreciation factor or LDF to help decide how much excess light to initially produce. This tries to factor in predicted LLD light loss over whole lamp life, but only uses averages. This approach still uses a number of fixtures which initially produce excess light, but later may not produce enough light. As can be appreciated, this results in added capital and energy costs initially, and added energy and maintenance costs thereafter (e.g. operating additional lamps at ROW over their entire operating lives, and having to replace more lamps over time). It also may result in a deficiency of light later. But this has been the conventional balance adopted by the state of the art.
The state of the art has, therefore, moved in the direction of developing and using higher wattage lamps, and intentionally designing in additional fixtures that produce an initial excess amount of initial light output for an application. This addresses part of the LLD issue, but not all of it. It does not address added cost (capital and operation). Therefore, there is room for improvement in the art.
There are also continuing attempts to make other improvements involving HID lighting. For example, improvements have been made in increasing the efficiency of lighting fixtures to direct more light from each lamp to the field, see, e.g., U.S. Pat. Nos. 4,725,934, 4,816,974, 4,947,303, 5,075,828, 5,134,557, 5,161,883, 5,229,681, and 5,856,721. But, the problem of light loss from lumen depreciation of HID lamps remains a problem in the art.
There are also circuits which enable selective dimming of lights. See, for example, Musco Corporation MULTI-WATT® system and U.S. Pat. No. 4,994,718. Capacitance is added or deleted to change light output from one or more lamps. However, this provides a user the option to select, at any time, between more or less light to the target. It does not address compensation for LLD.
Special ballasts have also been developed, particularly for fluorescent lamps, to try to keep light output from a lamp uniform over its life. However, these tend to be relatively complex, require significant interfacing components or circuitry with the lighting system, and therefore are relatively expensive and impractical. They also do not address the issues of composite lighting by sets of fixtures, as exists in lighting such as sports lighting or other composite area lighting. Therefore special ballasts of the type mentioned are generally considered too expensive for use in most lighting applications.